Halide gases, such as chlorofluorocarbon (CFC) gas, hydrochlorofluorocarbon (HCFC) gas, hydrofluorocarbon(HFC) gas, perfluorocarbon (PFC) gas, SF6 gas, and NF3 gas, have been used extensively in view of their chemical characteristics as a refrigerant, a blowing agent, a propellant, an electrically insulating gas, gas for metal refining, etchant gas or cleaning gas in semiconductor fabrication, and the like. However, it has been pursued since the late 80's to restrict or reduce the use of, or use substitutes for, CFC, HCFC, etc. because of their catalysis on destruction of the ozonophere and HFC, PFC, SF6, etc. because of their very powerful greenhouse effect.
Nevertheless because satisfactory substitutes have not been discovered as yet, there still remain some applications in which halide gases continue being used. For example, there is found no satisfactory substitute for electrically insulating gas for gas-insulated electrical equipment, such as gas-insulated switchgears, gas circuit breakers, and gas-insulated transformers. Hence in an attempt to reduce use of halide gases, electric power industries have developed replacement of pure halide gases for electric insulation with mixed gases comprising a halide gas, e.g., SF6 (sulfur hexafluoride), Freon or carbon tetrachloride, and a diluting gas such as N2 gas. SF6 gas is characterized by high dielectric strength upon being pressurized and by lower liquefying temperature than Freon or carbon tetrachloride gas, which enables use in low temperatures. Therefore, a mixed gas comprising SF6 gas and a diluting gas is a highly promising substitute as an electrically insulating gas.
A mixed gas of SF6 gas and a diluting gas also finds use as a protective cover gas during casting in magnesium foundries.
Semiconductor fabrication sites use halide gases, such as perfluorocarbon (PFC) gas, hydrofluorocarbon (HFC) gas, SF6 gas, and NF3 gas, as an etchant gas or a cleaning gas and emit mixed gases comprising the halide gas and a diluting gas such as N2 gas.
After released into the atmosphere, halide gases such as HFC, PFC, and SF6 sustain in the atmosphere without decomposition for an extremely long period of time because of their high chemical stability. If emitted into the atmosphere, however in a small amount, they will be accumulated in the atmosphere to give considerably serious influences to the global environment. Emission of halide gases into the atmosphere should therefore be minimized.
The electric power industry and the semiconductor industry have already set up definite targets for reducing use of halide gases and for controlling emission of halide gases, and various efforts have been made to achieve the targets. For example, Federation of Electric Power Companies (Japan) set the target of SF6 gas recovery rate at 97% during maintenance work and 99% at disposal of equipment by 2005. For guidelines of reuse, 97% by volume or higher purity is demanded for SF6 gas concentration in Japan. International Council on Large Electric Systems (CIGRE) WG 23.10 Task Force 01 demands 98% by volume or higher in Recycling Guide—Re-use of SF6 Gas in Electrical Power Equipment and Final Disposal, 1997.
Under these circumstances, an apparatus and a method for separating and recovering a halide gas from a halide gas-containing mixed gas in high concentration (at high purity) economically with minimized recovery loss are desired.
Halide gases being expensive, it would be extremely economical to recover them with high purity and reuse them.
Pressure cooling has been studied as a method of separating and recovering a halide gas from a mixed gas of a halide gas and other gas. However, the pressure cooling method generally needs high pressure and low temperature. Considering an extremely high pressure and an extremely low temperature required for achieving a high recovery rate of a halide gas with reduced recovery loss, it is practically difficult to recover a halide gas at high recovery rates. For example, H. Hama et al. report that treatment of a mixed gas having an SF6 content of 7% or less at 3.5 MPa and −50° C. resulted 0% liquefaction, proving virtual impossibility of SF6 gas recovery from a mixed gas having an SF6 gas content of 10% or less (8th International Symposium on Gaseous Dielectrics, Virginia Beach, June 22-I, “Application problems of SF6/N2 mixtures to gas insulated bus” (1998). They also revealed that the liquefaction rate (recovery rate) of even a mixed gas having 50% SF6 gas treated at 3.5 MPa and −10° C. does not reach 50%.
That is, the recovery rate of SF6 gas from an SF6 gas/diluting gas mixture used as an electrically insulating gas achievable by pressure cooling under practical conditions is about 50%. This means difficulty in separating and recovering SF6 gas at a high recovery rate with a reduced recovery loss.
JP-A-11-345545 proposes using a gas separation membrane selected from a polyimide membrane, a carbon membrane, and a zeolite membrane as an effective substitute for the pressure cooling method in the separation and recovery of SF6 gas. According to this method SF6 gas is separated and recovered as non-permeate gas of the gas separation membrane. It is true that SF6 gas has a smaller rate of permeation than other gas of a mixed gas (e.g., N2 gas), but this does not mean that SF6 gas is incapable of passing through a gas separation membrane at all. SF6 gas, while not much compared with other gas (e.g., N2 gas), also enters a permeate gas stream through a gas separation membrane together with the other gas. The SF6 in the permeate gas results in recovery loss. This recovery loss increases with an attempt to increase the purity of SF6 gas in the non-permeate. Therefore, it is difficult to separate and recover SF6 gas with a high concentration (high purity) at a high recovery rate.
JP-A-2000-140558 discloses a method and an apparatus for separating and recovering SF6 gas from an SF6-containing mixed gas by use of an aromatic polyimide separation membrane. The publication is silent to a specific process or a specific apparatus with which a halide gas can be recovered with a reduced recovery loss and with a high purity enough to be reused.
JP-A-2000-185212 discloses a method and an apparatus for separating and recovering a perfluorocompound gas from a perfluorocompound-containing mixed gas by use of an asymmetric carbon membrane obtained by carbonizing an asymmetric polyimide film. This publication has no mention of a specific process or a specific apparatus with which a halide gas can be recovered with a reduced recovery loss and with a high purity enough to be reused.
JP-A-10-128034 discloses a method of separating and recovering a fluorochemical gas from a mixed gas containing a diluting gas and the fluorochemical gas by use of gas separation membranes, wherein the non-permeate gas of a first gas separation membrane is led to a second gas separation membrane, and the non-permeate gas of the second gas separation membrane is collected as a separated and recovered fluorochemical gas.
JP-A-9-103633 proposes a method and an apparatus for separating and recovering a perfluorocompound gas from a gas mixture using a gas separation membrane made of a glassy polymer, wherein the non-permeate gas of a first gas separation membrane is led to a second gas separation membrane, and the non-permeate gas of the second gas separation membrane is collected as a separated and recovered perfluorocompound gas.
JP-A-10-298118 discloses a method and a system for separating and recovering a perfluorocompound gas from a gas mixture using gas separation membranes including a carbon sieve membrane, wherein the non-permeate stream of a first gas separation membrane is sent to a second gas separation membrane, and the non-permeate stream of the second gas separation membrane is collected as a separated and recovered perfluorocompound gas.
These methods using a plurality of gas separation membranes are suitable to recover a fluorochemical gas or a perfluorocompound gas with a higher purity from a mixed gas. However, the methods also involve the recovery loss problem due to passage of a fluorochemical gas or a perfluorocompound gas through the first gas separation membrane together with a diluting gas. Electrically insulating gas comprising a mixture of, for instance, SF6 gas and a diluting gas usually has a relatively high SF6 content ranging 3 to 60% by volume. When these methods are applied to the separation and recovery of SF6 gas from such an electrically insulating gas, it is difficult to separate and recover SF6 gas in high concentration (high purity) at high recovery rate since an unignorable amount of SF6 gas is emitted together with the permeate gas of the first gas separation membrane.
The technique using a gas separation membrane is superior to the pressure cooling method for the separation and recovery of a halide gas from a halide gas-containing mixed gas. In the light of the above-mentioned problem notwithstanding the superiority, it has been desired to develop an improved apparatus and an improved method capable of reducing the recovery loss of a halide gas and increasing the purity of the recovered gas to a level enough to be reused.